1. Field of the Invention
The present invention relates to a rotation detector for detecting the rotational speed and phase of a rotary member and, in particular, is directed to a rotation detector having particular applicability as a pulse generator for a video tape recorder.
2. Description of the Prior Art
Generally, a video tape recorder (VTR) of the helical scan type includes a rotary magnetic head which records and/or reproduces video signals from a record medium, such as a magnetic tape. The magnetic head is generally mounted on a guide drum about which the tape is at least partially wrapped and, more particularly, about the upper rotary drum of the guide drum. A drum servo circuit is provided for controlling the rotational speed and phase of the upper rotary drum, and consequently, of the head. In this regard, the helical scan video tape recorder is provided with a rotation detector, that is, a pulse generator, for detecting the rotational speed and phase of the upper rotary drum.
Two types of pulse generators are known, both of which include three iron cores upon which coils are wound, with two iron cores being arranged on a rotary member, such as the upper rotary drum, along an outer circumferential path coaxial with the rotational axis of the rotary member and with third iron core being arranged on the rotary member along an inner circumferential path which is coaxial with the outer circumferential path. Such VTRs are also generally constructed with a rotary transformer disposed within the inner circumferential path to supply recording signals to the magnetic head secured to the upper rotary drum and to receive reproduced signals from the magnetic head. However, because of space limitations, it is difficult to arrange all of the coiled iron cores and the rotary transformer in a limited area on the upper rotary drum.
Further, a problem results with such conventional pulse generators in that part of the magnetic flux from a magnet disposed along one of the outer and inner circumferential paths for use with the coiled iron cores also affects the magnetic flux received by a coil disposed along the other of the outer and inner circumferential paths, thereby leading to a decrease in the S/N ratio of the output pulses therefrom. This defect may be prevented or controlled by enlarging the distance between the outer and inner coaxial circumferential paths along which magnets, yokes and coils are provided. However, such enlargement of the distance therebetween results in the size of the detector and rotary drum being rendered unnecessarily larger.